1. Field of the Invention
This invention relates to a process for the switching-off of current conduction in a thyristor with the help of at least one additional semiconductor component connected in parallel to the thyristor wherein this additional component takes over the load current of the thyristor for a period of time which corresponds at least to the turn-off time of the thyristor, which in turn is related to the properties and the operational mode of the thyristor. This invention further relates to a semiconductor module for performing this process.
2. Description of the Prior Art
Thyristors are silicon semiconductor rectifier diodes which are used in many ways in signaling electronics as well as in high-performance electronics, for example, in control circuits, adjustable drives, inverted converters, rectifiers with controllable or adjustable output voltage, etc., owing to their long life, freedom from maintenance and insensitiveness to vibrations. The thyristor can be switched into a current-conductive state by means of a control impulse led to the gate. On the other hand, it is not possible to interrupt the local current flowing between the thyristor anode and cathode simply by means of a control signal applied to the gate. For this purpose, the load current must drop below the holding current which necessarily occurs at the end of each positive half wave in the case of an operation with alternating current and, for which purpose, additional quenching circuits are necessary for a controlled current interruption. Different designs of quenching or commutating circuits are known to the expert which are often more complex than the load circuit of the thyristor.
Therefore, a process has already been proposed for the switching-off of thyristors which can be carried out with considerably less circuit complexity. In this process, the load current of the thyristor is commuted to a component connected in parallel during the necessary release period. The component is preferably a semiconductor diode whose cathode is directly connected with the anode and whose anode is connected with the cathode of the thyristor over the secondary winding of a transmitter. In order to cut off the thyristor, a current impulse is produced in this secondary winding which flows through the diode and heterodynes the load current in the thyristor. This current impulse acts as charging current which floods the diode with charge carriers so that the diode takes over the pertinent backward current and the load current of the thyristor with the reversion of the current impulse effected by the circuit inductivity.
At least two conditions must be met to carry out this process. Firstly, the inductivity of the diode-thyristor circuit must be very small so that the backward current sets in through the diode before the blocking delay charge is recombined. Secondly, this charging current impulse must be considerably larger than the instantaneous value of the load current to be cut out so that the backward current can commutate this load current to the diode. These conditions and the circumstance that the charging current heterodynes the load current in the thyristor in the forward direction have limited the application of this process, so far, only to some special circuits.